This invention relates to improvements in a fluid flow meter for detecting a flow amount of gas, and more particularly to the fluid (air) flow meter suitable for detecting a flow amount of intake air of an automotive engine and the like.
In general, automotive engines or the like are arranged such that the flow amount of intake air of the engine is detected by an air flow meter or sensor, and then the injection amount or the like of fuel is determined in accordance with a detected value of the intake air flow amount, thereby forming air-fuel mixture having an appropriate air-fuel ratio upon mixing intake air and fuel
A typical one of air flow meters used for the above purpose is disclosed in Japanese Patent Provisional Publication No. 2000-2572 and arranged as follows: An air flow meter includes a casing installed to an intake air pipe in which intake air flows. The casing includes a main body section defining a board installation recess, and a peripheral wall section projecting from the main body section and surrounding the board installation recess. A part (abutting to the inside of the intake air pipe) of the peripheral wall section is cut out to form an installation plate insertion groove. An installation plate is provided including a board installation section disposed in the board installation recess, and an element installation section extending from the board installation section through the installation plate insertion groove to outside of the board installation recess. A circuit board is disposed on the board installation section of the installation plate. Electronic parts are provided to the circuit board. Additionally, an air flow sensor element is disposed on the element installation section of the installation plate so as to detect a flow amount of intake air.
In such an air flow sensor element, a temperature sensing resistor and a heater are formed on a silicon substrate by using metal thin film made of platinum or the like. The air flow sensor element is formed extending through the installation plate insertion groove into the board installation groove so that a part of the air flow sensor is located within the board installation groove. The part of the air flow sensor is connected to the circuit board to constitute a bridge circuit and the like. The temperature sensing resistor of the air flow sensor element is to be in contact with flow of intake air and cooled by intake air, under a condition where the temperature sensing resistor is heated by the heater. At this time, change in temperature (electrical resistance value) of the temperature sensing resistor is detected as a flow amount of intake air.
During operation of the engine, a part of intake air flowing the intake air pipe tends to penetrate into the board installation recess through the installation plate insertion groove. Accordingly, for example, a silicon-containing material gel is previously filled in the board installation recess and then solidified thereby covering the circuit board with this silicon-containing material, thus protecting the circuit board from dust, water and the like contained in intake air, in the above conventional technique disclosed in Japanese Patent Provisional Publication No. 2000-2572.
However, most automotive vehicles are equipped with an evaporative emission control system and/or an exhaust gas recirculating system. In the evaporative emission control system, evaporative gas or fuel gas generated within a fuel tank is flown into the intake air passageway to be mixed with intake air in order to prevent the evaporative gas from being released to the atmospheric air. The evaporative gas mixed with intake air is supplied to the engine to be combusted. In the exhaust gas recirculating system, a part of exhaust gas is recirculated through the intake air pipe into the engine in order to purify exhaust gas.
As a result of operation of the evaporative emission control system and/or the exhaust gas recirculating system, intake air flowing through the intake air pipe possibly contains the volatile (fuel) components of gasoline or light oil (diesel fuel). Consequently, when intake air penetrates into the board installation recess of the air flow meter and comes into contact with the silicon-containing material, there is the possibility that the silicon-containing material deteriorates owing to its swelling and melts to be flown out, because the silicon-containing material is low in durability against the volatile (fuel) components.
Thus, in the conventional technique, the silicon-containing material within the board installation recess may melt and flow out though the installation plate insertion groove so as to adhere onto the temperature sensing resistor and the like. This may make unstable the detection accuracy of the measured air flow amount while lowering the reliability of the air flow meter.
It is an object of the present invention to provide an improved fluid (air) flow meter which can effectively overcome drawbacks encountered in similar conventional fluid flow meters.
Another object of the present invention is to provide an improved fluid (air) flow meter using a fluid flow sensor element, which can stably detect the flow amount of a measured fluid at a high accuracy throughout a long time, while improving the reliability of the fluid flow meter.
A further object of the present invention is to provide an improved fluid (air) flow meter using a fluid flow sensor element, in which a silicon-containing material or filler filled in a board installation recess of a casing can be effectively prevented from being deteriorated by a component contained in a measured fluid.
An aspect of the present invention resides in a fluid flow meter comprising a casing installed to a conduit in which a measured gas flows. The casing includes a main body section defining a board installation recess, and a peripheral wall section projecting from the main body section and surrounding the board installation recess, a part of the peripheral wall section being cut out to form an installation plate insertion groove, the part abutting to an inside of the conduit. An installation plate is provided including a board installation section disposed in the board installation recess, and an element installation section extending from the board installation section through the installation plate insertion groove to an outside of the board installation recess. A circuit board is disposed on the board installation section of the installation plate, electronic parts being provided to the circuit board. A gas flow sensor element is disposed on the element installation section of the installation plate, the gas flow sensor element detecting a flow amount of the measured gas. A silicon-containing filler is disposed in the board installation recess of the casing to cover the circuit board, the silicon-containing filler being formed of a material containing silicon. Additionally, a fluorine-containing sealant is disposed in the board installation recess at a portion close to the installation plate insertion groove, the fluorine-containing sealant sealing the installation plate insertion groove so as to maintain the silicon-containing sealant inside the board installation recess, the fluorine-containing sealant being formed of a material whose major component is a polymer containing fluorine.
Another aspect of the present invention resides in an air flow meter for intake air of an automotive internal combustion engine. The air flow meter comprising a casing installed to an intake air pipe in which intake air flows. The casing includes a main body section defining a board installation recess, and a peripheral wall section projecting from the main body section and surrounding the board installation recess, a part of the peripheral wall section being cut out to form first and second end portions which are separate from each other to define therebetween an installation plate insertion groove, the part abutting to an inside of the conduit, the first and second end portions having respectively first and second edges which face each other and parallel with each other. An installation plate includes a board installation section disposed in the board installation recess, and an element installation section extending from the board installation section through the installation plate insertion groove to an outside of the board installation recess. A circuit board is disposed on the board installation section of the installation plate, electronic parts being provided to the circuit board. An air flow sensor element is disposed on the element installation section of the installation plate, the air flow sensor detecting a flow amount of intake air, the air flow sensor element including a temperature sensing resistor. A silicon-containing filler is disposed in the board installation recess of the casing to cover the circuit board, the silicon-containing filler being formed of a material containing silicon. A fluorine-containing sealant is disposed in the board installation recess at a portion close to the installation plate insertion groove, the fluorine-containing sealant sealing the installation plate insertion groove so as to maintain the silicon-containing sealant inside the board installation recess, the fluorine-containing sealant being formed of a material whose major component is a polymer containing fluorine. Additionally, a plate-shaped stopper member is connected to the casing and inserted in the installation plate insertion groove of the casing and fitted between the first and second edges of the respective first and second end portions of the peripheral wall section of the casing. The stopper member has a tip end in contact with the air flow sensor element. The temperature sensing resistor of the air flow sensor element and the silicon-containing filler are located respectively on opposite sides of the stopper member.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.